2,623 research outputs found
Planar Resonators for Metamaterials
This paper presents the results of an investigation into a combination of electric and magnetic planar resonators in order to design the building element of a volumetric metamaterial showing simultaneously negative electric and magnetic polarizabilities under irradiation by an electromagnetic wave. Two combinations of particular planar resonators are taken into consideration. These planar resonators are an electric dipole, a split ring resonator and a double H-shaped resonator. The response of the single resonant particle composed of a resonator with an electric response and a resonator with a magnetic response is strongly anisotropic. Proper spatial arrangement of these particles can make the response isotropic. This is obtained by proper placement of six planar resonators on the surface of a cube that now represents a metamaterial unit cell. The cells are distributed in space with 3D periodicity
Role of Strain on Electronic and Mechanical Response of Semiconducting Transition-Metal Dichalcogenide Monolayers: an ab-initio study
We characterize the electronic structure and elasticity of monolayer
transition-metal dichalcogenides MX2 (M=Mo, W, Sn, Hf and X=S, Se, Te) with 2H
and 1T structures using fully relativistic first principles calculations based
on density functional theory. We focus on the role of strain on the band
structure and band alignment across the series 2D materials. We find that
strain has a significant effect on the band gap; a biaxial strain of 1%
decreases the band gap in the 2H structures, by as a much 0.2 eV in MoS2 and
WS2, while increasing it for the 1T materials. These results indicate that
strain is a powerful avenue to modulate their properties; for example, strain
enables the formation of, otherwise impossible, broken gap heterostructures
within the 2H class. These calculations provide insight and quantitative
information for the rational development of heterostructures based on these
class of materials accounting for the effect of strain.Comment: 16 pages, 4 figures, 1 table, supplementary materia
Ferromagnetic insulating state in tensile-strained LaCoO thin films
With local density approximation + Hubbard (LDA+) calculations, we
show that the ferromagnetic (FM) insulating state observed in tensile-strained
LaCoO epitaxial thin films is most likely a mixture of low-spin (LS) and
high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM
states, including the intermediate-spin (IS) state (\textit{metallic} within
LDA+), which consists of IS Co only, and the insulating IS/LS mixture state,
the HS/LS state is the most favorable one. The FM order in HS/LS state is
stabilized via the superexchange interactions between adjacent LS and HS Co. We
also show that Co spin state can be identified by measuring the electric field
gradient (EFG) at Co nucleus via nuclear magnetic resonance (NMR) spectroscopy
Analysis of dynamic inlet distortion applied to a parallel compressor model
An investigation of surge was conducted by using a parallel compressor model of the J85-13 compressor implement on an analog computer. Surges were initiated by various types of dynamic disturbances in inlet pressure. The compressor model was less sensitive to disturbances of short duration, high frequency, and long duration where the compressor discharge pressure could react. Adding steady distortion to dynamic disturbances reduced the amount of dynamic disturbance required to effect surge. Steady and unsteady distortions combined linearly to reduce surge margin
Magnetic structure and orbital ordering in BaCoO3 from first-principles calculations
Ab initio calculations using the APW+lo method as implemented in the WIEN2k
code have been used to describe the electronic structure of the
quasi-one-dimensional system BaCoO3. Both, GGA and LDA+U approximations were
employed to study different orbital and magnetic orderings. GGA predicts a
metallic ground state whereas LDA+U calculations yield an insulating and
ferromagnetic ground state (in a low-spin state) with an alternating orbital
ordering along the Co-Co chains, consistent with the available experimental
data.Comment: 8 pages, 9 figure
Cooperative Effect of Electron Correlation and Spin-Orbit Coupling on the Electronic and Magnetic Properties of Ba2NaOsO6
The electronic and magnetic properties of the cubic double perovskite
Ba2NaOsO6 were examined by performing first-principles density functional
theory calculations and analyzing spin-orbit coupled states of an Os7+ (d1) ion
at an octahedral crystal field. The insulating behavior of Ba2NaOsO6 was shown
to originate from a cooperative effect of electron correlation and spin-orbit
coupling. This cooperative effect is responsible not only for the absence of
orbital ordering in Ba2 NaOsO6 but also for a small magnetic moment and a weak
magnetic anisotropy in Ba2NaOsO6
Relative phase stability and lattice dynamics of NaNbO from first-principles calculations
We report total energy calculations for different crystal structures of
NaNbO over a range of unit cell volumes using the all-electron
full-potential (L)APW method. We employed both the local-density approximation
(LDA) and the Wu-Cohen form of the generalized gradient approximation (GGA-WC)
to test the accuracy of these functionals for the description of the complex
structural behavior of NaNbO. We found that LDA not only underestimates the
equilibrium volume of the system but also predicts an incorrect ground state
for this oxide. The GGA-WC functional, on the other hand, significantly
improves the equilibrium volume and provides relative phase stability in better
agreement with experiments. We then use the GGA-WC functional for the
calculation of the phonon dispersion curves of cubic NaNbO to identify the
presence of structural instabilities in the whole Brillouin zone. Finally, we
report comparative calculations of structural instabilities as a function of
volume in NaNbO and KNbO to provide insights for the understanding of
the structural behavior of KNaNbO solid solutions.Comment: Accepted for publication in Physical Review
Spin-state crossover and hyperfine interactions of ferric iron in MgSiO perovskite
Using density functional theory plus Hubbard calculations, we show that
the ground state of (Mg,Fe)(Si,Fe)O perovskite, a major mineral phase in
the Earth's lower mantle, has high-spin ferric iron () at both the
dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site
iron undergoes a spin-state crossover to the low-spin state (), while
the A-site iron remains in the high-spin state. Our calculation shows that the
B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied
by a noticeable volume reduction and an increase in quadrupole splitting,
consistent with recent X-ray diffraction and M\"ossbauer spectroscopy
measurements. The volume reduction leads to a significant softening in the bulk
modulus, which suggests a possible source of seismic velocity anomalies in the
lower mantle.Comment: 11 pages, 4 figures, 1 tabl
Effect of doping and pressure on magnetism and lattice structure of Fe-based superconductors
Using first principles calculations, we analyze structural and magnetic
trends as a function of charge doping and pressure in BaFeAs, and
compare to experimentally established facts. We find that density functional
theory, while accurately reproducing the structural and magnetic ordering at
ambient pressure, fails to reproduce some structural trends as pressure is
increased. Most notably, the Fe-As bondlength which is a gauge of the magnitude
of the magnetic moment, , is rigid in experiment, but soft in calculation,
indicating residual local Coulomb interactions. By calculating the magnitude of
the magnetic ordering energy, we show that the disruption of magnetic order as
a function of pressure or doping can be qualitatively reproduced, but that in
calculation, it is achieved through diminishment of , and therefore
likely does not reflect the same physics as detected in experiment. We also
find that the strength of the stripe order as a function of doping is strongly
site-dependent: magnetism decreases monotonically with the number of electrons
doped at the Fe site, but increases monotonically with the number of electrons
doped at the Ba site. Intra-planar magnetic ordering energy (the difference
between checkerboard and stripe orderings) and interplanar coupling both follow
a similar trend. We also investigate the evolution of the orthorhombic
distortion, as a function of , and find that in the
regime where experiment finds a linear relationship, our calculations are
impossible to converge, indicating that in density functional theory, the
transition is first order, signalling anomalously large higher order terms in
the Landau functional
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